Opioids are among the most used analgesics for moderate-to-severe chronic pain management.1 Opioids can be given via virtually every route of administration, which affords significant dosing flexibility. Although oral administration generally is the preferred route when possible, opioids may be administered via oral, transmucosal (sublingual, buccal, intranasal), transdermal (passive, iontophoresis), spinal (intrathecal, epidural), rectal, topical, and intra-articular routes.1 Generally, it is not possible to define maximum opioid doses for individual patients because the dose is adjusted based on the intensity of pain and level of opioid tolerance from previous and regular exposure.2 When pain is not adequately controlled despite increasing doses of the opioid or because the presence of intolerable adverse effects influences compliance, switching opioids or changing the route of administration may be indicated.3,4 Therefore, variable analgesic potencies of different opioids may be considered, with specific attention paid to analgesic effect, and the potential for overdosing or underdosing, the worst outcomes of which could be death or withdrawal, respectively.

Although opioids are one of the oldest therapeutic classes of medications, conversion from one or more opioids to another remains haphazard and variable. Considerable confusion persists about the process of opioid conversion when medications are rotated, or switched, which can be necessary to realize the most favorable balance of therapeutic effects and side effects in patients who require opioid analgesic therapy as a component of overall pain management.5 Opioid rotation, defined as “a change in opioid drug or route of administration with the goal of improving outcomes,”6 begins with the selection of a safe and effective starting dose for the new opioid.3 Once initiated, the new therapy must be individualized via dose titration and treatment of adverse effects. Given the large differences in potency among opioids, receptor variability and affinities, polymorphic receptor variation, and individual tolerability, the selection of a starting dose must be conservatively estimated, giving careful consideration to the relative potency between the existing opioid(s) and the new one.7 Ideally, clinicians should switch with an initial dose that does not result in adverse effects or abstinence but maintains efficacy. In clinical practice, however, determination of optimal initial doses when rotating opioids is a challenge.

Clinicians generally calculate doses for opioid rotation using equianalgesic dose tables, but published data for conversion frequently are inconsistent.1 Primary and secondary literature, company package inserts, and online sources for opioid conversions have conflicting equianalgesic dosing guidelines.4 Some conversion tables refer to older studies using single-dose designs rather than chronic dosing at steady state.1 The extrapolation of the results from single-dose studies to the context of chronic opioid dosing is, therefore, not valid.1 Moreover, the various tables often indicate different conversion ratios,4 which further complicate conversion calculations for opioids.

Aside from potential drug interactions and compromised organ function, providers must consider interpatient and intrapatient polymorphic variability, genetics, and physiological differences.8 Important considerations in converting from one opioid to another also include demographic factors, comorbidities, and drug interactions. In considering which specific opioid should be tried next, clinicians should weigh a patient’s history of any drug sensitivities or experiences with specific drugs; drug characteristics that may increase or decrease safety or efficacy, given the patient’s clinical status; drug characteristics that may offer previously unrealized benefits unrelated to pain relief (eg, convenience, improved adherence, less reliance on oral administration, or access to a regular non-opioid drug in a combination product); and problems related to financial issues or insurance coverage.7

If a clinician selects an opioid that requires enhanced knowledge for safe prescribing, such as methadone or transmucosal fentanyl, they should ensure that their skills are adequate, obtain appropriate consultation, or refer to persons with expertise in prescribing these drugs. To reduce the risk of unintentional overdose when pain intensity may be changing quickly or when rapid titration is needed, opioid rotation in the setting of acute pain management should employ a short-acting opioid, rather than an extended-release formulation or methadone.7

Although opioid rotation is a common practice, there are substantial limitations in the standard of care. To reduce the risk of unintentional overdose, the conversion ratio calculated for a patient undergoing opioid rotation should be adjusted based on clinical assessment of risk. Since it is unlikely that a standard algorithm could be developed to meet all of the considerations outlined herein for every patient, opioid rotation remains both art and science, requiring experience and skill.7

At the time of manuscript submission, there were eight equianalgesic conversion calculators available online. Unfortunately, there are substantial differences in availability, quality, complexity, features, and limitations among the eight. While calculators can help prevent computational errors, clinical judgment and individualization of treatment are necessary to switch a patient safely and effectively from one or more opioids to another.

Comparison of Online Calculators

Methods

Primary and secondary literature, manufacturers’ package inserts, and online sources for opioid dose conversions have conflicting equianalgesic dosing guidelines and recommendations. This disparity creates considerable confusion about the process of opioid conversion, rotation, and switching. Since providers and pharmacists often use questionable equianalgesic conversion tables or easy-access opioid conversion calculators as tools for converting opioids, we present an updated comparison.

This project aimed to compare and contrast the various online opioid conversion calculators, identify the mathematical disparities in conversion, compare automated conversions against manual calculations, reveal potential risks to the end user, and make recommendations to health care providers for practical and safe approaches when predicting opioid conversions. We hypothesized that there is a wide disparity among available online calculators, which presents potential risks to patients, providers, and pharmacists.

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